High accuracy, low flow valves are used in a variety of applications. Such applications include high pressure gas chromatography, fractional distillation, and the manufacture of semi-conductor wafers. The manufacture of semi-conductor wafers presents specific problems in managing gas flow in epitaxal reactors which manufacture today's semi-conductor products. As is well known by those of ordinary skill in the art, a silicon wafer for a semi-conductor device is built up in layers from a semi-conductor wafer substrate, typically manufactured from silicon dioxide. The degree to which the silicon dioxide conducts electricity in the presence of an electrical field is determined by the quantity of "impurities" which are driven into the surface of the substrate. These "impurities" are in fact desirable elements and their presence or absence must be carefully controlled. Furthermore, the circuits which are imbedded in the semi-conductor wafer are deposited in layers on top of the substrate. Subsequent etching and redeposition steps, including a final metalization layer results in a complex circuit imbedded in the semi-conductor wafer.
All of the above described deposition steps are typically accomplished by chemical vapor deposition in a high temperature oven better known as an epitaxal reactor. In all cases, the different layers are deposited by vapor deposition from materials in a gaseous state. In order to control the deposition, and to more precisely control the growth of the layers which are built up on the substrate surface the following variables must be carefully controlled: the temperature of the reactor, the flow rate of the selected gas over the substrate, and the time of exposure. It is well known that the degree to which the above steps can be achieved quickly and accurately significantly increases yield thereby reducing the cost of producing a semi-conductor product.
Before the development of the present invention, it was necessary to control the flow of gasses through the epitaxal reactor manually by turning valves connected to an appropriate gas generator such as a pressurized cylinder. In addition to the inaccuracies inherently present in a manual system considering operator variability, it is also known that the epitaxal reactor must be "shut down" while the gas flow valves are adjusted. This time consuming, labor intensive and inherently variable process is one major source of inefficiency in the semi-conductor manufacturing industry.
Thus, a need exists for a high accuracy, automatic gas flow control valve which can be operated under electronic or computer control so as to eliminate the deficiencies noted above.